This invention concerns the storage and conveyance of thermal energy at low temperature.
A lot of research has been done into change of state phenomena, particularly solid/liquid and liquid/solid state changes, for products with high latent heat, with the aim of reaching a solution that can be applied industrially.
Apart from the high latent heat storage potential, such products must fulfill numerous other requirements, such as lowest possible cost, availability, simplicity of preparation, absence of danger during handling, as regards toxicity, flammability and corrosion. Finally, variations in specific volume during the change of state must be taken carefully into account.
Change of state phenomena, particularly the solid/liquid state change, have been studied from this viewpoint, for simple or compound bodies, simple or complex salts, saline hydrates, eutectic mixtures, and various mixtures such as paraffins.
Experiments have shown the frequency of phenomena involving delay in initiation of the change of state in relation to the theoretical temperature defined for the phenomenon. These delays are often long, and usually vary for the same product depending on the conditions under which the change of state occurs.
The length of the delay depends on numerous factors such as the type and size of the system, type and distribution of impurities, method of preparation of the product and treatments which it has undergone since preparation. In fact, the change of state delay has to be investigated for every product in every operating situation.
In the article entitled "Solar energy storage", published in "Ashrae Journal" of September 1974, pages 40 to 43, by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, 345 East 47th Street, New York NY 10017, Doctor Maria Telkes reports on a large number of change of state experiments done on various products.
Calcium chloride hexahydrate CaCl.sub.2, 6 H.sub.2 O, although inexpensive, has the disadvantage of contributing to equilibria, producing four separate crystalline forms with different conversion points. Under certain conditions the product is corrosive.
Sodium carbonate decahydrate Na.sub.2 CO.sub.3,10 H.sub.2 O also participates in equilibrium reactions producing several crystalline forms.
Na.sub.2 HPO.sub.4,12 H.sub.2 O and Ca (NO.sub.3).sub.2 allow simpler operating layouts to be used, but are quite expensive.
Suitable nucleating agents have been investigated for all these products, to encourage the liquid/solid change and reduce delay in change.
Sodium thiosulphate Na.sub.2 S.sub.2 O.sub.3,5 H.sub.2 O has also been experimented, but is quite expensive and, more important, unstable.
Sodium sulphate decahydrate Na.sub.2 SO.sub.4,10 H.sub.2 O, which is a very cheap product, available in large quantities and which is safe to use, would appear, according to research, to offer the best prospect for industrial application.
The effectiveness of borax Na.sub.2 B.sub.4 O.sub.7,10 H.sub.2 O as nucleating agent has been established, apparently being related to the structure of its crystalline form, the parameters of which are close to those of sodium sulphate decahydrate.
During successive heat storage/release cycles, it has been observed that an anhydrous solid fraction Na.sub.2 SO.sub.4 with a density of 2 separates out during the first part of the cycle and tends to settle at the bottom of the container, whereas during the second part of the cycle it should interact with the liquid phase to ensure optimum efficiency of conversion.
When the solid fraction of Na.sub.2 SO.sub.4 settles as a deposit, there is a serious drop in the scale of reaction exhanges during the next cycle, and consequently a drop in heat release because of the reduction in interface between liquid and solid phases.
The use of mechanical agitators, or of very wide, low tanks have been suggested as a way of avoiding this segregation of Na.sub.2 SO.sub.4. But such methods are in fact not very effective, nor practicable.
Doctor Telkes begins by proposing nucleation of the continuous medium by dispersing a nucleating agent with very low water solubility, such as borax Na.sub.2 B.sub.4 O.sub.7,10 H.sub.2 O, and she also suggests the addition of thickeners to prevent Na.sub.2 SO.sub.4 from settling down.
Various organic thickening agents have been used to produce formation of a gel, which blocks the movements and particularly the downward movement of Na.sub.2 SO.sub.4 particles. These agents, such as starch, must be mixed with additive stabilizing agents such as formol.
This combination of methods has not produced an entirely satisfactory solution, since beyond 20 cycles the various additives become ineffective.
A silica gel, formed in the actual product, has proved to impede the movement or transfer of the product, since the whole mixture becomes too viscous.
A paraffin-water emulsion cannot be used unless a dispersing agent, such as silicaceous earths, is added.
Experiments on these last two processes have shown that borax loses its nucleating effect when in the presence of silica products.
The present invention overcome these difficulties by ensuring very fine fragmentation of the active substance mixed with the nucleating agent, without increasing the liquid phase viscosity. The invention concerns a product suitable for storage and conveyance of heat energy, constituted by a dispersion of at least one salt with high latent heat storage and release capacity when undergoing a change of state, in an oil to which at least one emulsifying agent has been added, this salt being thoroughly mixed with at least one nucleating agent. The end product, when ready for storage, must contain almost no liquid water in the solution.
In this new product, at least one salt is preferably Na.sub.2 SO.sub.4,10 H.sub.2 O and at least one nucleating agent is preferably borax, and a recommended emulsifying agent contains lanoline.
One method of manufacturing this new product to store and convey thermal energy consists of the following succession of steps:
at least one salt comprising a hydrate having a high latent heat storage and release capacity when undergoing a change of state is dissolved in the form of a monophasic solution, and at least one nucleating agent is dispersed finely in this solution; PA1 this solution is emulsified in the form of droplets in an oil to which at least one emulsifying agent has been added; PA1 the emulsion is placed in a dry atmosphere which is kept at a temperature above the change of state point of the hydrate, until the concentration in the droplets is within a few percent of the specified hydrate concentration; PA1 the emulsion is cooled down to a temperature far enough below the change of state point for this change to take place completely a first time; PA1 the emulsion is reheated to the change of state point and thermal energy is supplied at this temperature until the temperature of the emulsion begins to rise above the change of state point; PA1 the emulsion is recooled to a temperature far enough below the change of state point for the change to take place completely a second time; PA1 the cycle comprising heating to the change of state point followed by cooling to a temperature far enough below this point for the change to take place completely is repeated at least once. PA1 thermal energy is supplied to the product at a temperature at least equal to the change of state point until the temperature of the product begins to rise above this point; PA1 the temperature of the product is allowed to evolve, and when it reaches the "release" point, which is at most equal to the change of state point, heat begins to be released, the temperature of the product returns to the change of state point, and thermal energy is emitted for use as required. PA1 thermal energy is supplied to the product at a temperature at least equal to the change of state point until the temperature of the product begins to rise above this point; PA1 the temperature of the product is allowed to evolve, and when it reaches the minimum temperature selected for regulation of the enclosed space, the product is placed quickly in contact with the cold source of a thermal appliance until nucleation of the entire product is initiated in all emulsion droplets; PA1 the temperature of the product is allowed to rise to the change of state point, and the released thermal energy is used to raise the temperature of the enclosed space until the maximum selected temperature is reached; PA1 heat release is halted by placing the product in contact with the hot source of the thermal appliance.
In one recommended process for manufacturing a product according to the invention, suitable for storage and conveyance of thermal heat, the salt is sodium sulphate, the hydrate Na.sub.2 SO.sub.4,10 H.sub.2 O of which has a solid/liquid change of state point of 32.degree. C., at least one nucleating agent is borax, the temperature of the dry atmosphere in which the product is placed to bring the water content close to that of the hydrate is approximately 40.degree. C., and the liquid/solid change point, to ensure that the change takes place completely, is approximately -50.degree. C. during the first cycle.
Using a product prepared in the way described above, any macroscopic segregation of anhydrous Na.sub.2 SO.sub.4 is made impossible, because of the extreme fragmentation of the medium. Consequently, this product retains its properties for a very long, almost indefinite period. Industrial application of the product therefore becomes possible.
In this new process for storage and release of thermal energy using the product described above:
In one recommended embodiment, the product is constituted by an emulsion, in an oil to which an emulsifying agent has been added, of a solution of Na.sub.2 SO.sub.4 close to the hydrate Na.sub.2 SO.sub.4,10 H.sub.2 O, in which borax has been finely dispersed as nucleating agent, the change of state equilibrium point is approximately +32.degree. C., and the likeliest heat release point is approximately +8.degree. C.
In a heat storage and release process in which the product is kept in thermal balance with an enclosed space, the temperature of which must be regulated between a maximum and minimum level:
Heat energy supplied at the hot source of the thermal appliance helps to heat the enclosed space.
In this process, the product used is preferably constituted by an emulsion, in an oil to which an emulsifying agent has been added, of a solution of Na.sub.2 SO.sub.4 close to the hydrate Na.sub.2 SO.sub.4,10 H.sub.2 O, in which borax is finely dispersed as nucleating agent, the change of state point is approximately +32.degree. C., and the heat release point is approximately +8.degree. C.
According to this process, thermal energy can be stored in the product at a temperature above the change of state point, after which this product can be conveyed through unlagged pipes, for example in a floor at a temperature of slightly above 10.degree. C., and the heat energy can be released at a remote point by initiating nucleation.
It will be easier to understand the invention from the following description of one of the possible sets of experiments performed during preparation of the product and application of this product, this description being given by way of illustration, but not of limitation.
Emulsions containing droplets with a concentration of approximately 1 mole Na.sub.2 SO.sub.4 to 10 moles water, i.e. 44 g Na.sub.2 SO.sub.4 to 100 g solution, cannot be prepared directly. First, an emulsion of an under-saturated solution containing 30 g Na.sub.2 SO.sub.4 and 5 g borax to 100 g solution is prepared in a paraffin oil vehicle combined with an emulsifying agent which encourages a water-in-oil emulsion. This gives a creamy emulsion with a viscosity of approximately 14 poises at 20.degree. C., containing droplets of approximately 1 micron in diameter. The dispersed solution can then be evaporated, for example at 30.degree. C., without precipitation of salt in the droplets, until a concentration similar to that of the solid hydrated salt is achieved, such as 41%.
This emulsion is placed in a Perkin Elmer D.S.C.2. enthalpimeter and subjected to continuous cooling (5.degree. C..times.min.sup.-1) during which predominant crystallizations are observed around -46.degree. C. During the following heating (5.degree. C..times.min.sup.-1) slight eutectic fusion and transition at a temperature of +32.degree. C. are observed.
During the next cooling (5.degree. C..times.min.sup.-1) supersaturation is observed to break between +12.degree. C. and -7.degree. C. with two crystallization peaks at +7.degree. C. and -2.5.degree. C.
These two exothermal peaks represent calorific energy corresponding to approximately 70% of that of the endothermal peak observed at 32.degree. C. during heating.
Initial crystallization at -46.degree. C. shows a high level of supersaturation, proving that the borax itself has also remained supersaturated. It should be noted that when there is no borax the supersaturation break occurs at approximately -41.degree. C. The low eutectic fusion shows that not all the water has been included in the salt in the form of crystallization water at the point of supersaturation break.
The appearance during the second cooling of two crystallization peaks reveals the nucleating effect of the borax, which has precipitated during the first cooling and partly dissolved during reheating. A convenient quantity of borax is used for this condition to be met. The nucleating effect of borax has been confirmed by studying an emulsion containing no borax in which a very high degree of supersaturation is noted during the second cooling, the same as that observed during the first cooling.
The existence of two crystallization peaks during cooling shows different behaviour for two populations of droplets. This result could be attributed to the presence of droplets of different diameters, except that in this case the phenomenon should also be observed during the first crystallization. It is likelier that borax does not have the same effect in all droplets. The nucleating effect of borax microcrystals would then comply with a distribution law governed by temperature, which would be reflected precisely by the form of the peaks. The borax also seems to be less active when dispersed in droplets than in macroscopic phase. However, in this case all the liquid crystallizes as soon as the first nucleations resulting from a number of active borax centres, which may be quite low, have been initiated. In emulsion, they concern too few droplets to be detectable. Borax obtained after high supersaturation may also be less active.
Thirty successive cooling/heating cycles were performed between +40.degree. C. and -15.degree. C. Phenomena are found to occur with significant increase in storage, the ratio of crystallization areas observed during thirtieth and first cycles being 1.16.
Increased crystallization is also observed around +7.degree. C., which could reflect an improvement in the nucleating capacity of borax. Good conservation of the emulsion during the thirty cycles is also evidence, because coalescences would be bound to cause reduction in efficiency.
Coalescences may have a twofold effect: by increasing the size of dispersed droplets, they could make earlier non-significant nucleations on very fine dispersions effective, but they could also locally, and later generally, cause a return to macroscopic volumes and harmful anhydrous sulphate segregations.
The following example describes an application of the product prepared set forth above.